The present invention relates to measurement of optical properties of materials, and, more particularly, to precision optical measurement of refractive index, concentration and temperature of materials.
The precise measurement of concentration in liquids is important in fields such as chemical analysis and processing, diagnostics, semiconductor manufacturing, waste inspection, and measurement of liquid diffusion coefficients. Measurement of liquid temperature is also important, as is determination of the refractive index for both liquids and non-liquid materials. As such, a variety of techniques have been developed to measure concentration, temperature and/or refractive index. Bergman et al. developed a fiber-optic probe to measure salinity distribution in liquids, as described in T. L. Bergman, F. P. Incropera and W. H. Stevenson, xe2x80x9cMiniature Fiber-Optic Refractometer for Measurement of Salinity in Double-Diffusive Thermohaline Systems,xe2x80x9d 56 Rev. Sci. Instrum. 291-96 (1985) and T. L. Bergman, D. R. Munoz, F. P. Incropera and R. Viskanta, xe2x80x9cMeasurement of Salinity Distributions in Salt-Stratified, Double-Diffusive Systems by Optical Deflectometry,xe2x80x9d 57 Rev. Sci. Instrum. 2538-41 (1986). Other techniques include a planar laser-induced fluorescence technique, as described in A Lozano, S. H. Smith, M. G. Mungal, and R. K. Hanson, xe2x80x9cConcentration Measurements in a Transverse Jet by Planar Laser-Induced Fluorescence of Acetone,xe2x80x9d 32 AIAA Journal 218-21 (1993), and an invasive heat-marker method as described in V. A. Vink, G. A. Sokolov, and Yu. S. Fomochev, xe2x80x9cMeasurement of the Concentration of Flowing Liquid Solutions,xe2x80x9d 58 Journal of Applied Chemistry of the USSR 357-59 (1985). Interferometric techniques are described in T. Konishi, S. Naka, A. Ito and K. Saito, xe2x80x9cTransient Two-Dimensional Fuel-Concentration Measurement Technique,xe2x80x9d 36 Applied Optics 8815-19 (1997), T. A. Wilson and W. F. Reed, xe2x80x9cLow Cost, Interferometric Differential Refractometer,xe2x80x9d 61 Am. J. Phys. 1046-48 (1993) and R. J. Harris, G. T. Johnston, G. A. Kepple, P. C. Krok and H. Mukai, xe2x80x9cInfrared Thermooptic Coefficient Measurement of Polycrystalline ZnSe, ZnS, CdTe, CaF2, and BaF2, Single Crystal KCI, and TI-20 Glas,xe2x80x9d 16 Applied Optics 436-38 (1977), and a phase-locked-loop ultrasonic method is described in K. Ikeda, xe2x80x9cUltrasonic Measurement of Concentration in Solutions by a Phase-Locked Loop Method,xe2x80x9d 36 Jpn. J. Appl. Phys. 3180-83 (1997).
Refractometers are routinely used to evaluate the refractive index to determine the concentration of a liquid mixture, as described in J. E. Geake, xe2x80x9cLinear Refractometers For Liquid Concentration Measurement,xe2x80x9d Chemical Engineer 305-08 (1975). Still other techniques reported in the literature for accurately measuring the refractive index of solids and gases include the minimum deviation method as set forth in I. H. Malitson, xe2x80x9cRefractive Properties of Barium Fluoride,xe2x80x9d Journal of the Optical Society of America 628-32 (1964) and B. C. Platt, H. W. Icenogle, J. E. Harvey, R. Korniski and W. L. Wolfe, xe2x80x9cTechnique for Measuring the Refractive Index and Its Change with Temperature in the Infrared,xe2x80x9d 65 Journal of the Optical Society of America 1264-66 (1975), the use of a Littrow prism as described in E. D. McAlister, J. J. Villa and C. D. Salzberg, xe2x80x9cRapid and Accurate Measurements of Refractive Index in the Infrared,xe2x80x9d 46 Journal oft he Optical Society of America 485-87 (1956) and A. R. Hilton and C. E. Jones, xe2x80x9cThe Thermal Change in The Nondispersive Infrared Refractive Index of Optical Materials,xe2x80x9d 6 Applied Optics 1513-17 (1967), Brewster angle techniques as described in I. K. Smirnov, Y. G. Polyakov and G. N. Orlov, xe2x80x9cArrangement for Measurement of Index of Refraction and Thickness of Transparent Dielectric Films by an Optical Method,xe2x80x9d Journal of the Optical Society of America 546-47 (1980), and others.
Examples of other techniques are set forth in S. M. Chernov, K. K. Zhilik and P. G. Rabzonov, xe2x80x9cDetermination of the Index Refraction of Liquids and Gases in Capillaries,xe2x80x9d 37 Journal of Applied Spectroscopy (English Translation of Zhurnal Prikladnoi Spektroskopii) 1069-72 (1982), L. A. Danisch, xe2x80x9cRemoving Index of Refraction Constraints in the Optical Measurement of Liquid level,xe2x80x9d Fiber Optics and Laser Sensors X 268-79 (1992) and D. R. Lide, Ed., CRC Handbook of Chemistry and Physics (CRC Press, Boca Raton, 1998). All of these techniques, however, suffer from one or more of the following shortcomings: direct contact is required with the material being measured, poor resolution is noted, complicated and expensive components are required, systems are physically large and difficult to operate, or a visual, subjective analysis of the data is relied upon.
In view of the foregoing deficiencies of currently known techniques for measurement of concentration and temperature in materials, including liquids, a need clearly exists for an apparatus and method for precision, non-contact measurement of concentration and temperature of a material, such as a liquid, where the apparatus and method does not require complicated and expensive components, is compact, easy to operate, and does not rely on visual, subjective readings of measurement data.
The present invention, which substantially overcomes the shortcomings of the currently known techniques, provides a method for determining the concentration and temperature of a transparent liquid. The method includes the steps of causing the liquid to be contained in a vessel having a transparent entrance side and a transparent exit side; causing a beam of light to impinge on the entrance side; and then calculating the refractive index of the liquid using Snell""s law. In the step of causing the liquid to be contained in the vessel, the vessel can have an entrance side and an exit side with a known angular relationship therebetween, and can be immersed in known surroundings. In the step of causing the beam of light to impinge on the entrance side of the vessel, the impingement can be at an angle xcex8i with the normal to an outer surface of the entrance side. The beam can then pass through the material, and then through the exit side, from which it exits at an angle xcex8e with respect to the normal to an outer surface of the exit side impinged by the light beam.
The calculation of the refractive index of the material using Snell""s law can be based on the angles xcex8i and xcex8e, and can be done by applying Snell""s law at the interface between the surroundings and the entrance side, the interface between the entrance side and the material, the interface between the material and the exit side, and the interface between the exit side and the surroundings.
The present invention also provides a method for determining the concentration of a given component (e.g., a solute) in a sample of a multi-component liquid mixture (e.g., a solution). The method includes the step of determining the refractive index of the sample of the multi-component liquid mixture as described above and then comparing the determined refractive index of the sample of the multi-component liquid mixture to predetermined data relating different concentrations of the given component of the multi-component liquid mixture to corresponding values of the refractive index of the multi-component liquid mixture. In this manner, the concentration of the given component of the sample of the multi-component liquid mixture can be determined from the refractive index of the sample of the multi-component liquid mixture determined as described above.
The present invention yet further provides a method for determining a change in concentration of a given component of a sample of a multi-component liquid mixture from an initial concentration of the given component which corresponds to an initial refractive index of the sample of the multi-component liquid mixture, at an initial sample temperature. The method includes determining an initial and a subsequent refractive index of the sample of the multi-component liquid mixture in the manner described above, and then determining a change in the refractive index by subtracting the initial refractive index from the subsequent refractive index. The method further includes measuring the initial and subsequent temperatures of the sample of the multi-component liquid mixture and then determining the change in the concentration of the given component in the sample of the multi-component liquid mixture from the initial concentration according to the following approximate formula:
xe2x80x83xcex94C≅(xcex94nxe2x88x92(∂n/∂T)xcex94T)(∂n/∂C)xe2x88x921,xe2x80x83xe2x80x83(1)
where:
xcex94n is the change in the refractive index of the sample,
xcex94C is the change in the concentration of the given component in the sample of the multi-component liquid mixture from the initial concentration,
∂n/∂T is partial derivative of index of refraction, n, of the multi-component liquid mixture, with respect to temperature, determined from known data (can be evaluated at sample temperature and is approximately constant for a xcex94T up to 10-15xc2x0 C., and can be treated as a function of temperature for larger temperature changes),
xcex94T is the difference between the subsequently measured temperature of the sample and the initial temperature, and
∂n/∂C is partial derivative of index of refraction, n, of the multi-component liquid mixture, with respect to concentration of the given component, evaluated from known data in a region near the initial concentration and near the sample temperature.
Also provided is a method for determining the temperature of a given sample of a liquid, which can be either a pure liquid or a multi-component liquid having a substantially constant concentration of the components. The method includes the step of determining the refractive index of the sample of the liquid as initially described above, and then comparing the refractive index of the sample of the liquid to predetermined data relating different temperatures of the liquid to corresponding values of the refractive index of the liquid. In this way, the temperature of the sample of the liquid can be determined which corresponds to the refractive index of the sample of the liquid which was determined as described above.
The present invention yet further provides an apparatus for determining the concentration of a solute and the temperature of a light transmitting liquid. The apparatus includes a vessel which contains the liquid, a light source, and a light beam position sensor. The vessel has a planar entrance side and a planar exit side with a known angular relationship therebetween, and is immersed in known surroundings. The light source is located so as to cause a beam of light to impinge on the entrance side at an angle xcex8i with respect to the normal to the outer surface of the entrance side where the beam impinges, to pass through the material, and to then pass through the exit side, from which it exits at an angle xcex8e with respect to the normal to the outer surface of the exit side where the beam exits. The sensor determines the exit angle xcex8e, thus permitting the refractive index of the material to be determined, based on the angles xcex8i and xcex8e, by applying Snell""s law. The application of Snell""s law occurs at the interface between the surroundings and the entrance side, the interface between the entrance side and the material, the interface between the material and the exit side, and the interface between the exit side and the surroundings. The apparatus includes a computer which is suitably programmed to calculate the refractive index of the liquid, and to determine the unknown concentration or temperature of the liquid sample based on comparison with predetermined data.
The present invention thus provides methods and an apparatus capable of carrying out precision refractive index, concentration and temperature measurement of liquids. The apparatus and methods overcome the disadvantages of prior systems and methods. In particular, the present method and apparatus permit real time, non-invasive, remote measurements with high resolution and can be carried out using simple and inexpensive components. Further, the apparatus is compact and easy to operate. Yet further, measurements can be based on readily quantifiable and measurable parameters, rather than visual, subjective parameters. Real-time remote monitoring of liquid component concentration, such as real-time mixture monitoring, precision solute and contaminant analysis, and measurement of diffusion coefficients of components of a multi-component liquid mixture, for example, are all possible with the present invention.
These and other features and advantages of the present invention will be pointed out in the following specification, taken in connection with the accompanying drawings, and the scope of the invention will be set forth in the appended claims.